Need help,
searching for noise causing lost steps.
My second CNC is a YOOCNC/carving-cnc CNC3040 that I’m using as a mechanical base.
I had lost steps with their larger 6040 too but proper grounding and the removal of the noisy cooling-water pump solved that.
Now the 3040 is also loosing steps, however lpt-breakout and x,z,y-driver are one integrated board, controlling a A-driver, getting (probably noisy) 24V from an integrated VFD-supply+power regulator board and giving it a pulse signal to control the spindle speed.
Any idea where I could start looking for the cause of the lost steps in such a tightly integrated system of 3 boards?
…and what I could do about it except to rip it out and install a separate 24V supply, USB-control board and 4 dedicated stepper driver boards?
First of all I would give ferrite cores and better isolation to all cables or use twisted and shielded cables. Enclose the spindle. A radio or telephone can cause white noise, too. Try to put an active monitor-box to your power-source without the machine to locate the noise. If you can hear the white noise the problem is the power source.
It’s all in the same box and mostly on the same PCB. not many places for the usual shielding, grounding and ferrite cores.
How would you enclose a spindle? It needs to be attached to the frame and be unenclosed to do any milling. 
Do you mean the VFD? It’s also the 24V power supply for the steppers and is inside the same box as everything else.
put a box/cage/enclosure around the spindle (top + bottom open of course, ;D ) and mount it on Z. But the problem sounds more like a bad power supply. Any chance to place additional Elkos on the board?
The spindle VFD IS the power supply for the steppers.
noisy 24V?
I could meassure the 24V output and if it’s noisy add a ceramic (not Elko) capacitor between GND and +24V but we’re talking about 4x1.5A at 24V here.
I’m not sure the power supply can handle the additional load introduced by the capacitor.
I guess it can.
other causes?
I’m not sure about other places to looks for the cause given the described setup above.
LPT levels/timing?
Since the breakout board is not generating it’s own clock fro step+dir, the LPT part may also be at fault. (timing due to software generated pulses on old Computer or 3.3V logic levels on LPT cable when 5V or 15V levels are expected).